Inkjet printers of the type discussed above are commonly referred to as continuous inkjet printers, as distinct from drop-on-demand inkjet printers, because ink drops continue to be provided even when they are not required for printing and, usually, a jet of ink will be provided continuously while the printer is in operation. Although unwanted ink drops may enter the gutter under their own momentum, it is necessary to provide suction at the gutter or not far behind it, in order to suck away the ink that has entered the gutter. This is particularly the case if the printer is to be operated in an arrangement such that the gutter is no higher than the intended destination for the recovered ink, so that it is impossible to transport the ink away from the gutter by gravity. One consequence of this use of suction is that the line conveying ink from the gutter to the suction source will normally also carry air, and in most commercial inkjet printers the volume of air flowing down this line is much greater than the volume of ink.
Various proposals have been made to modify the airflow in the line between the gutter and the suction source.
JP-A-02-106354 proposes that the cross-sectional area of the opening of the gutter should be smaller than the cross-sectional area of the pipe leading from the gutter to a suction pump, so that the airflow is faster near the gutter opening and it flows rapidly into the pipe, and is sucked at a lower speed along the pipe. This is intended to reduce the amount of solvent that evaporates from the ink.
GB 1553720 proposes an arrangement in which the gutter is connected by a short length of relatively large diameter tubing to a nearby separator, in which the ink and air that have entered the gutter are allowed to separate. A further relatively large diameter tube conveys air from the separator to a vacuum source. A second, relatively small diameter tube carries the ink from the separator to an evacuated ink return tank. By separating the ink from the air shortly after they have entered the gutter, this arrangement seeks to minimise the evaporation of solvent from the ink.
It is also known, e.g. from the Linx 5000 printer, to fit an insert into the line from the gutter to the suction source having a reduced internal cross-sectional area compared with the rest of the line, to act as a flow restrictor and thereby reduce the amount of air that is sucked into the gutter.
JP-A-07-060993 proposes that, because the level of suction required varies depending on the height of the printhead relative to the printer body containing a vessel for recovered ink, a device is provided in the line from the gutter to a suction pump in order to control the speed of flow along the line. When the printer is installed, this device is adjusted until just before the point that ink is no longer reliably collected from the gutter. Alternatively, the device may be adjusted beforehand, using a flow meter. Alternatively, a device having a fixed effect on the flow (by changing the bore of the line) may be used, and several such devices may be provided so that an appropriate one is selected and put in place before the printer is installed.
WO99/62717 proposes that the suction applied to the gutter should be intermittent or pulsed, either by interrupting the operation of an electric suction pump or by opening and closing a valve in the duct between the gutter and a venturi pump, in order to reduce the amount of solvent stripped out of the ink by the air which is sucked in through the gutter, and also to reduce aeration of the ink in the reservoir where it is collected.
WO02/100645 and JP-A-59-073957 both propose that recovered ink should be accumulated at the gutter until the amount of ink triggers a switch which operates a suction pump briefly to suck away the accumulated ink. In WO02/100645 this is intended to avoid various problems arising from sucking a large amount of air in through the gutter. In JP-A-59-073957 this is intended to allow a smaller and less expensive ink recovery pump to be used.
JP-A-57-084855 concerns an inkjet printer with a movable carriage, in which unwanted ink drops are collected by a gutter but recovery of ink from the gutter is not performed during printing but only when the carriage is stopped or is moving in a return direction. This is in order to prevent the gutter suction from affecting the printed characters.
EP 0805040 proposes a system in which the gutter is connected by a return line to an evacuated ink tank. The tank is initially set to a high vacuum level, creating “slug flow” in the return line, causing wide pressure swings in the return line as frothy slugs and liquid alternately travel past a pressure transducer. The tank vacuum is lowered until the pressure in the return line stops fluctuating, indicating a switch to “bubble flow”, in which air in the return line takes the form of individual separate bubbles rather than frothy slugs.
JP-A-2002-154225 proposes that printing data is used to calculate the number of ink drops passing to the gutter per unit time interval, and that the speed of an ink recovery pump should be controlled accordingly, so that the pump generates a larger negative pressure when the flow of ink into the gutter is greater. This is intended to reduce the amount of solvent lost from the ink.
Alternative methods of reducing the loss of solvent from the ink, not affecting the path or the flow conditions of air down the line from the gutter are also known. For example, it is known to chill the air that has passed along the gutter line, as disclosed for example in JP-A-01-247167, in order to condense out evaporated solvent.